In recent years the endothelial cell glycocalyx has been shown to play a pivotal role in microvessel function by modulating rheology, permeability, and leukocyte-endothelial cell interactions. We now propose to extended those observations with the hypothesis that the glycocalyx is a dynamic structure, which is regulated and that pathophysiological stimuli can cause its degradation and disorganization, which will contribute to enhanced intimal permeability, platelet adhesion, accelerated white cell binding, and emigration of white cells from the vasculature. In vivo video microscopy allows us to follow changes in the microcirculation in response to stimuli, and microperfusion allows us to mark the glycocalyx and to selectively treat small, localized segments of the microcirculation. We propose two experimental aims which will place the role of the glycocalyx in endothelial cell function on a much firmer footing, and which will set the groundwork for an understanding of the role for the glycocalyx in pathophysiology. Specific Aim #1 - to test and explore the hypothesis that the glycocalyx is an adaptive component of the vascular wall, and that it is a key potential site for damage. The following questions will be answered to test the hypothesis. 1. Do vasoactive substances alter the glycocalyx by modifying wall shear stress? 2. Do inflammatory stimuli exert a common set of effects on the size and permeability of the glycocalyx? Specific Aim #2 - to test the hypothesis that one of the key sequences of events in response to ischemia/reperfusion is the activation of adenosine receptors and their primary and/or secondary effects on the glycocalyx. The following questions will be answered to test the hypothesis. 1. Can the effects of ischemia/reperfusion on the glycocalyx be reduced by activation of the adenosine A2A receptor? 2. Can the effects of I/R be mimicked by activation of A3 receptors? 3. Are the detrimental effects of large doses of adenosine mediated though actions on mast cells, leukocytes, or endothelial cells? The proposal is based on the use of recently developed highly potent and selective adenosine blockers, and a group of genetically engineered animals. The experiments offer the potential for understanding a new level of microvascular regulation, and for developing strategies to understand the intracellular signaling that leads to modification of the glycocalyx.